Techniques described in Patent Documents 1 to 4 are such that a gas chamber is formed around a second land portion of the piston, and during an initial period of the explosion and expansion stroke a high-pressure combustion gas from above the piston is introduced into and held in this gas chamber through gas passage holes provided in a cylinder inner surface, whereby the piston is supported from a thrust side by this high-pressure combustion gas introduced and held, to thereby reduce frictional resistance between the piston and the cylinder inner surface.